1. Field of the Invention
The invention relates to charge injection devices (CIDs) for sensing and imaging radiation, the CIDs having either linear or two dimensional arrays of sensing sites. More particularly, the invention relates to a novel readout circuit in which the integration time may be controlled to suit the application or radiation conditions. The invention has particular application to IR sensing in which the CID array may be required to respond to both low and very high level background and target radiation with minimal degradation in performance.
2. Prior Art
Radiation integrating detectors usually operate with an integration time such that background and dark current carriers fill a small fraction of the saturated charge capacity, leaving a large fraction of the capacity for target signal. However, some scenes have large backgrounds such as sunlit clouds, or have extended bright targets such as would occur at the end game of a missile flight. These can then cause saturation problems.
Previous attempts to increase the charge storage capacity of InSb CIDs, which are of application to IR sensing, have had only limited success. The saturated charge of the site is given by
Qsat=(KEo/t) AV.sub.I
where
K is the dielectric constant of the insulator PA1 Eo is the permittivity of free space PA1 t is the insulator thickness PA1 A is the gate area PA1 V.sub.I is the effective readout or "Inject" voltage.
Attempts to increase the Qsat by the use of thinner oxide or an increased injection voltage have not been successful because of breakdown effects. Increasing the Qsat by increasing gate area has had moderate success in scanned arrays. In that case, the gate area can be extended in the scan direction by addition of opaque storage gate area, keeping the optical aperture of the site fixed.
Another method of increasing Qsat includes ramping of the bias levels during integration so as to cause some background charge to replace the bias charge. This method has limited application to cases when the dynamic range within the scene is not too large.
Another method of increasing the Qsat is the use of buffer gates as a guard ring to lessen breakdown effects. This method can increase Qsat by a factor of two.